Patent GB-681571-A (GB)
Title: Improvements relating to automatic voltage control systems
Inventor: SHAW NORMAN HARRY
IPC: H02P9/32
Filed: 1950-04-28
Abstract: 681,571. Automatic voltage control systems. ENGLISH ELECTRIC CO., Ltd. April 20, 1951 [April 28, 1950], No. 10523/50. Class 38 (iv). The output voltage of an alternator is controlled by varying the excitation of the exciter in a corrective sense in accordance with the difference between the rectified primary and secondary currents of a saturating transformer supplied from the alternator output, the secondary of the transformer being connected to a capacitor so that the secondary current varies linearly with primary voltage whilst the primary current varies non-linearly. The two characteristic curves have a point of intersection which is selected as the normal operating point. The field 1a of the star-connected threephase alternator 1 is excited by the exciter 3 and sub-exciter 4 arranged as in Specification 644,867 and having a control field 4a energized according to the difference between the output voltages of rectifiers 8b, 8c. These output voltages are proportional to the primary and secondary currents respectively of the saturating transformer 8a, the primary of which is connected (effectively) to the potential transformer 5, whilst the secondary is connected across a capacitor 8e. The potential transformer 5 is connected to an auto-transformer 6 in series with impedances R1, Y1, B1, supplied from current transformers Rc, Yc, Bc. The current transformers in the R and B phases form a negative sequence filter which produces a negative sequence voltage across the associated loading impedances equal and opposite to the negative sequence voltage measured in the potential transformer 5. Further, in order to obtain correct sharing of the wattless current in parallel working, a voltage proportional to the current per phase is derived from transformer Yc. This voltage is vectorially at approximately 90 degrees to the voltage measured across the Y and B phases when the power factor is 0.8 lagging. Thus, the resulting voltage applied to transformer 6 is the positive sequence component of the line voltage when the power factor of the alternator is maintained at 0.8 lagging. This voltage is fed to the measuring transformer 8a. via a compensating network consisting of a series reactor 7b and a tuned parallel circuit comprising the capacitor 7a, reactor 7c and the A.C. winding 9a of the magnetic amplifier 9. The tuned circuit compensates for frequency variations by varying the voltage drop in reactor 7b in a corrective sense. The magnetic amplifier has three additional functions. (a) In order to prevent further reduction of alternator excitation when the leading wattless current approaches the stability limit, a control winding 9f is energized in accordance with the difference between the outputs of rectifiers 10b, 10c. Rectifier 10b is energized by transformer winding 10a1 in accordance with the line voltage. Rectifier 10c is energized by transformer 10e in accordance with the line voltage plus a component derived from the current transformer Rc. A reactor 10f effects a 30 degrees phase-shift on the latter to cancel a similar phase-shift in the main transformer 2. The voltage applied to rectifier 10b is normally greater than that applied to 10c. When the leading wattless current exceeds the limiting value the winding 9f is energized, thereby increasing the voltage drop in reactor 7b permitting the alternator voltage to rise. (b) In order to increase the accuracy of the system, a winding 9d is energized in accordance with the voltage across the output side of the frequency compensating circuit, and a winding 9b is energized in accordance with the voltage applied to the control field 4a. (c) In order to improve the stability, a winding 9c is energized from a feedback transformer 8f in accordance with the rate of change of the current in control field 4a.